While not necessarily limited thereto, the present invention is particularly adapted for use in a baghouse of the type in which dust-laden air enters a lower plenum chamber and then flows upwardly through apertures in an upper wall of the plenum chamber and into a plurality of vertically-elongated tubular filter bags which separate the dust from the air as it flows through the fabric walls of the bags. The bottom of each filter bag is open and is secured to the periphery of a thimble which extends upwardly from the upper wall of the plenum chamber and surrounds an associated one of the aforesaid apertures.
In the past, thimbles of the type described above have been constructed such that a relatively sharp annular edge is presented between the upper wall of the plenum chamber and each aperture formed therein. When gas in the plenum chamber exits through an aperture of this type with a sharp edge, part of the gas will approach along the center line of the exit path through the aperture; however another part comes toward the aperture along the underside of the upper wall of the plenum from all directions. The part approaching along the underside of the upper wall has momentum toward the center of the aperture; and to turn this part of the gas around a sharp corner (i.e., through 90.degree.) theoretically would require an infinite pressure gradient which, of course, is impossible. In actual practice, the radially-inflowing gas breaks away from the sharp corner in a turbulent flow pattern and makes a more or less gradual turn into the aperture. The result is a gas stream of smaller diameter than the aperture, a higher velocity for a given flow than would be the case if the outflow were at fullhole diameter, and a relatively high pressure drop across this point with a resultant pressure loss in the system.